The present invention relates to an electrically conductive material comprising a polymer of a compound having conjugated double bonds, which is provided on a specific base material, and to a secondary battery using this type of electrically conductive material.
It is known that polymers having conjugated double bonds in the main chain, such as polyacetylene, poly-p-phenylene, polythienylene, polypyrrole, polyaniline, and poly-p-phenylene-vinylene, are remarkably improved in electric conductivity when they are treated with a P- or N-type doping agent such as arsenic pentafluoride, antimony pentafluoride, iodine, bromine, sulfur trioxide, n-butyllithium, or sodium naphthalene, whereby they are changed from an insulator to a semiconductor or a conductor. These electrically conductive materials (so-called "electrically conductive polymers") are obtained in the form of powder, grain, bulk, or film, which is used either as such or after molding thereof in accordance with the purpose of use thereof. They are now under investigation as to the application thereof to a wide variety of fields involving not only functional elements such as an antistatic material, an electromagnetic wave shielding material, a photoelectric conversion element, an optical memory (holographic memory), and various sensors but also a display element (electrochromism), a switch, various hybrid materials (transparent conductive film, and the like), various terminal equipment, and a secondary battery.
However, this type of electrically conductive polymer is generally poor in moldability and processability. Particularly in order to obtain a film form of an electrically conductive polymer, a special process must be adopted. Today, known films of such electrically conductive polymers include a polyacetylene film which is generally prepared by blowing an acetylene gas against a glass wall coated with a polymerization catalyst to form a film and peeling the film from the glass wall, and polypyrrole and polythienylene films which are prepared by forming a film on an electrolysis electrode according to an electrochemical oxidation reaction (electrolytic oxidation polymerization) and peeling the film from the electrode.
Among the above-mentioned conventional electrically conductive polymer films, the polyacetylene film disadvantageously is so unstable in air as to undergo progressive oxidative deterioration, and has a low mechanical strenght, while the polypyrrole and polythienylene films and the like obtained by the above-mentioned electrolytic oxidation polymerization disadvantageously have their film size restricted by the size of the electrolysis electrode, and involve complicated steps and a high cost.
Further, Journal of Electronic Materials, Vol. 13, No. 1, pp. 211-230 (1984) revealed an electrically conductive material prepared by immersing a filter paper in 0.01M aqueous HCl containing FeCl.sub.3.6H.sub.2 O, bringing the filter paper into contact with pyrrole vapor or immersing the filter paper in a pyrrole solution to effect gas-phase or solution polymerization of the pyrrole on the filter paper. It further revealed an electrically conductive material prepared by bringing a pyrrole vapor into contact with a filter paper after immersion thereof in a solution of FeCl.sub.3.6H.sub.2 O--C.sub.2 H.sub.5 OH to effect gas-phase polymerization of the pyrrole into polypyrrole on the filter paper.
However, the former, namely the electrically conductive material prepared by the method involving immersion of a filter paper in 0.01M aqueous HCl containing FeCl.sub.3.6H.sub.2 O, contains water and disadvantageously undergoes drastic reduction in electric conductivity when dried. Thus, this electrically conductive material can be used only in a wet state (hydrous state). This presents a problem such that this material cannot be used, for example, as the electrode material of a secondary battery of the non-aqueous electrolytic solution system in reality. Further, in this electrically conductive material, iron compounds used for the polypyrrole formation remains as an impurity without being removed. The presence of this impurity presents problems of providing low performance and limited use and application of the electrically conductive material when it remains as it is due to its low electrical conductivity. On the other hand, the latter, namely the electrically conductive material prepared by the method involving immersion in a solution of FeCl.sub.3.6H.sub.2 O--C.sub.2 H.sub.5 OH, has an electric conductivity as low as 1/1000 of that of the above-mentioned material prepared by the method involving immersion in 0.01M aqueous HCl containing FeCl.sub.3.6H.sub.2 O, thus presenting a problem of being notably poor in performance as the electrically conductive material.
On the other hand, there has recently been proposed a secondary battery prepared by using an electrically conductive polymer as mentioned above as the electrode material.
Although such an electrically conductive polymer usually has a slight electric conductivity as described above, the electric conductivity thereof can be dramatically increased by doping since it can be doped with a dopant such as any one of various anions and cations, or can be undoped. In constituting a secondary battery with such an electrically conductive polymer as the electrode material, an electrically conductive polymer capable of being doped with anions is used as the anode material, and/or an electrically conductive material capable of being doped with cations is used as the cathode material, while a solution containing a dopant as mentioned above is used as the electrolytic solution. Thus, there can be produced a secondary battery capable of charging and discharging via electrochemically reversible doping and undoping.
Known electrically conductive polymers of the kind as described above include the aforementioned polymers having conjugated double bonds in the main chain, such as polyacetylene, poly-p-phenylene, polypyrrole, polythienylene, polyaniline, and poly-p-phenylene-vinylene. In an instance of polyacetylene, it is used as the electrode material for at least one of the anode and the cathode, while anions such as BF.sub.4.sup.-, ClO.sub.4.sup.-, SbF.sub.6.sup.- or PF.sub.6.sup.-, or cations such as Li.sup.+, Na.sup.+ or R.sub.4 N.sup.+ (wherein R represents an alkyl group) are employed to constitute an electrochemically reversible system capable of doping and undoping.
These electrically conductive polymers are obtained in the form of powder, grain, bulk, or film. In the case of using a powdery, grainy, or bulky form of an electrically conductive polymer as the electrode material in constituting a secondary battery with a non-aqueous electrolytic solution or a solid electrolyte, there is needed a step of press-molding the polymer into an electrode either as such or after addition of an adequate electrically conductive material for improving the electric conductivity and/or a thermoplastic resin for improving the mechanical strength of the resulting electrode. In this respect, the use of a film form of an electrically conductive polymer provides, for example, such a characteristic feature that the film can be only punched with a predetermined size into an electrode to considerably facilitate the electrode production.
Known examples of such an electrically conductive polymer film include not only polyacetylene, polypyrrole, and polythienylene films as described before, but also composite electrically conductive films obtained by coating a base material such as a PET film with a solution containing an oxidizing agent and a polymer binder and bringing the resulting base material into contact with a vapor of pyrrole, aniline, or the like to form a layer of an electrically conductive polymer film on the base material.
However, in the case of using a conventional electrically conductive polymer film as mentioned above as a battery electrode material constituting a secondary battery, a polyacetylene film quite disadvantageously undergoes polymer deterioration due to slight amounts of oxygen and water present in the battery, leading to a poor performance of the electrode, and causes, for example, a rapid increase in charging voltage and a decrease in charging and discharging efficiency during cycles, leading to a shortened cycling life span. Further, there have arisen problems such that the film is liable to be oxidized with oxygen contained in a working atmosphere, leading to a difficult and complicated production of electrodes, and that the preservability of electrodes is poor due to grave deterioration of the materail by oxidation.
In the case of using a polythienylene or polypyrrole film prepared by the electrochemical oxidation polymerization reaction, not only is the size of the film restricted by the size of the electrolysis electrode, but also a complicated production process and a need for a special production apparatus are involved, thus leading to a high battery production cost. Further, since a difficulty is encountered in obtaining a thick and uniform film, combined use of this film as a battery electrode with a collector involves such problems that the contact of the film with the collector may become poor during charging and discharging cycles, and that the battery reaction may occur concentrately in a portion of the electrode, thus causing deterioration in battery performance.
In the case of using a composite electrically conductive film as mentioned above, since the polymer binder is used in order to keep the oxidizing agent on the base material, an electrically conductive polymer obtained by the polymerization reaction is in the form of a compositie electric conductive made of a mixture of a polymer of pyrrole or aniline with the polymer binder. This decreases the concentration of the polymer of pyrrole or aniline having an electric conductivity in the electrically conductive polymer. Thus, when it is used as an electrode material, a problem of poor performance arises due to the disadvantageous reduction in the effective polymer concentration since the same performance as that in the case of using, for example, a conventional electrically conductive polymer film as mentioned above cannot be attained even is desired.